P0300 misfire help

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Tmoney10

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2004 suburban z71
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I just got this truck a month ago. I changed new sparkplugs ac Delco.
I had a knock sensor code also when I bought the truck, prior owner said he replaced the mad sensor also.
I ran 2 full tanks of e85 then I got a p0300 misfire. And a knock sensor code.

Stuff I've done so far
new coil packs, sparkplug wires, air filter, fuel filter. I put 87 octane with half a tank left of e85 and I out seafoam in the gas tank.

I checked all the wires and all grounds. I even pulled the 02 sensor to see if cats were clogged no luck truck still misfires...


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Ran 2 tanks of e85, got p0300...

My last guess is a clogged fuel injector?
 

OR VietVet

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Why are you not addressing the knock sensor code?

GM P0300 Causes
(* = Most Common)

  • Faulty or worn spark plugs *
  • Faulty spark plug wires or coils *
  • Distributor failure *
  • Faulty fuel injector *
  • Vacuum leak
  • Low fuel pressure
  • Camshaft sensor defective
  • Crankshaft sensor defective
  • Engine timing off
  • Leaking head gasket
  • Low engine compression
  • Poor quality fuel
 
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Tmoney10

Tmoney10

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Knock se.sor code was a pending one. I'm gon a swap both of those out and new intake manifold gaskets next..
 
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Tmoney10

Tmoney10

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So I replaced these spark plugs less then 500 miles ago brand new.... Cyl 6 and 7 had these all the others are fine... I replaced the plug but still getting misfire on 6 cyl
PXL_20210620_221606861.jpg
 

Fless

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Do fuel pressure checks and compression tests before you pull it apart. If you have a coolant pressure tester do that too. Narrow down possibilities before wrench.

Post up those compression test numbers for all cylinders, as well as the fuel pressure values.

Also curious about where you're sourcing your spark plugs.
 

corvette744

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So I replaced these spark plugs less then 500 miles ago brand new.... Cyl 6 and 7 had these all the others are fine... I replaced the plug but still getting misfire on 6 cyl
View attachment 305056
I agree those are toast,you said you had a po300 random misfire detected.Now you replaced plugs and you said it still misfiring on 6 cyl p0306 or you think your down 2 cylinders .Def do a compresion test immediately.If it passes that like was said coolant pressure test.Next leakdown test good luck.
 
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